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Continuous Probability Distributions

Chapter 07

McGraw-Hill/Irwin

LEARNING OBJECTIVES

LO 7-1 List the characteristics of the uniform distribution.

LO 7-2 Compute probabilities by using the uniform distribution.

LO 7-3 List the characteristics of the normal probability distribution.

LO 7-4 Convert a normal distribution to the standard normal distribution.

LO 7-5 Find the probability that an observation on a normally distributed random variable is between two values.

LO 7-6 Find probabilities using the Empirical Rule.

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The Uniform Distribution

The uniform probability distribution is perhaps the simplest distribution for a continuous random variable.

This distribution is rectangular in shape and is defined by minimum and maximum values.

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LO 7-1 List the characteristics of the uniform distribution.

The Uniform Distribution – Mean and Standard Deviation

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LO 7-1

The Uniform Distribution – Example

Southwest Arizona State University provides bus service to students while they are on campus. A bus arrives at the North Main Street and College Drive stop every 30 minutes between 6 A.M. and 11 P.M. during weekdays. Students arrive at the bus stop at random times. The time that a student waits is uniformly distributed from 0 to 30 minutes.

Draw a graph of this distribution.

Show that the area of this uniform distribution is 1.00.

How long will a student “typically” have to wait for a bus? In other words, what is the mean waiting time? What is the standard deviation of the waiting times?

What is the probability a student will wait more than 25 minutes?

What is the probability a student will wait between 10 and 20 minutes?

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LO 7-2 Compute probabilities

using the uniform distribution.

The Uniform Distribution – Example

1. Graph of this distribution.

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LO 7-2

The Uniform Distribution – Example

2. Show that the area of this distribution is 1.00.

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LO 7-2

The Uniform Distribution – Example

3. How long will a student “typically” have to wait for a bus? In other words, what is the mean waiting time?

What is the standard deviation of the waiting times?

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LO 7-2

The Uniform Distribution – Example

4. What is the probability a student will wait more than 25 minutes?

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LO 7-2

The Uniform Distribution – Example

5. What is the probability a student will wait between 10 and 20 minutes?

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LO 7-2

Characteristics of a
Normal Probability Distribution

It is bell-shaped and has a single peak.

It is symmetrical about the mean.

It is asymptotic: The curve gets closer and closer to the X-axis but never actually touches it.

The arithmetic mean, median, and mode are equal

The total area under the curve is 1.00.

The area to the left of the mean = area right of mean = 0.5.

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LO 7-3 List the characteristics of the normal probability distribution.

The Normal Distribution – Graphically

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LO 7-3

The Family of Normal Distribution

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Equal Means and Different Standard Deviations

LO 7-3

The Family of Normal Distribution

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Different Means and Standard Deviations

LO 7-3

The Family of Normal Distribution

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Different Means and Equal Standard Deviations

LO 7-3

The Standard Normal
Probability Distribution

The standard normal distribution is a normal distribution with a mean of 0 and a standard deviation of 1.
It is also called the z distribution.
A z-value is the signed distance between a selected value, designated X, and the population mean , divided by the population standard deviation, σ.
The formula is:

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LO 7-4 Convert a normal distribution to the standard normal distribution.

Areas Under the Normal Curve

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LO 7-4

The Normal Distribution – Example

The weekly incomes of shift foremen in the glass industry follow the normal probability distribution with a mean of $1,000 and a standard deviation of $100.

What is the z-value for the income, let’s call it X, of a foreman who earns $1,100 per week? For a foreman who earns $900 per week?

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LO 7-5 Find the probability that an observation on a normally distributed random variable is between two values.

Normal Distribution – Finding Probabilities

In an earlier example, we reported that the mean weekly income of a shift foreman in the glass industry is normally distributed with a mean of $1,000 and a standard deviation of $100.

What is the likelihood of selecting a foreman whose weekly income is between $1,000 and $1,100?

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LO 7-5

Normal Distribution – Finding Probabilities

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LO 7-5

Normal Distribution – Finding Probabilities Using the Normal Distribution Table

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LO 7-5

Finding Areas for z Using Excel

The Excel function

=NORMDIST(x,Mean,Standard_dev,Cumu)

=NORMDIST(1100,1000,100,true)

generates area (probability) from

Z=1 and below

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LO 7-5

Normal Distribution – Finding Probabilities (Example 2)

Refer to the information regarding the weekly income of shift foremen in the glass industry. The distribution of weekly incomes follows the normal probability distribution with a mean of $1,000 and a standard deviation of $100.

What is the probability of selecting a shift foreman in the glass industry whose income is:

Between $790 and $1,000?

Excel Function: =NORMDIST(1000,1000,100,true)-NORMDIST(790,1000,100,true)

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LO 7-5

Normal Distribution – Finding Probabilities using the Normal Distribution Table

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LO 7-5

Normal Distribution – Finding Probabilities (Example 3)

What is the probability of selecting a shift foreman in the glass industry whose income is:

Less than $790?

Excel Function: =NORMDIST(790,1000,100,true)

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LO 7-5

Normal Distribution – Finding Probabilities Using the Normal Distribution Table

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LO 7-5

Normal Distribution – Finding Probabilities (Example 4)

What is the probability of selecting a shift foreman in the glass industry whose income is:

Between $840 and $1,200?

Excel Function: =NORMSDIST(2.0)-NORMSDIST(-1.6)

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LO 7-5

Normal Distribution – Finding Probabilities Using the Normal Distribution Table

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LO 7-5

Normal Distribution – Finding Probabilities (Example 5)

What is the probability of selecting a shift foreman in the glass industry whose income is:

Between $1,150 and $1,250

Excel Function: =NORMSDIST(2.5)-NORMSDIST(1.5)

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LO 7-5

Normal Distribution – Finding Probabilities Using the Normal Distribution Table

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LO 7-5

Using z in Finding X Given Area – Example

Layton Tire and Rubber Company wishes to set a minimum mileage guarantee on its new MX100 tire. Tests reveal the mean mileage is 67,900 with a standard deviation of 2,050 miles and that the distribution of miles follows the normal probability distribution. Layton wants to set the minimum guaranteed mileage so that no more than 4 percent of the tires will have to be replaced.

What minimum guaranteed mileage should Layton announce?

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LO 7-5

Using z in Finding X Given Area – Example

Set the minimum guaranteed mileage (X) so that no more than 4 percent of the tires will be replaced.

Given Data:

µ = 67,900

σ = 2,050

X = ?

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LO 7-5

Using z in Finding X Given Area – Example

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LO 7-5

Using z in Finding X Given Area – Example

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LO 7-5

Using z in Finding X Given Area – Excel

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LO 7-5

The Empirical Rule

About 68 percent of the area under the normal curve is within one standard deviation of the mean.
About 95 percent is within two standard deviations of the mean.
Practically all is within three standard deviations of the mean.

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LO 7-6 Find probabilities using the Empirical Rule.

The Empirical Rule – Example

As part of its quality assurance program, the Autolite Battery Company conducts tests on battery life. For a particular D-cell alkaline battery, the mean life is 19 hours. The useful life of the battery follows a normal distribution with a standard deviation of 1.2 hours.

Answer the following questions.

1. About 68 percent of the batteries failed between what two values?

2. About 95 percent of the batteries failed between what two values?

3. Virtually all of the batteries failed between what two values?

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LO 7-6

The Empirical Rule – Example

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LO 7-6

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